The first line of the MM entry on ghosts is: "A ghost is the soul of a once-living creature, bound to haunt a specific location, creature, or object that held significance to it in its life."I disagree with some of the above posters about ghosts being souls. I think that's another "depends on the undead" situation. A ghost could easily be a remnant of the spirit left on the material plane for one reason or another, perhaps an incredibly powerful feeling left behind after death.
The first line of the MM entry on ghosts is: "A ghost is the soul of a once-living creature, bound to haunt a specific location, creature, or object that held significance to it in its life."
Your campaign may vary, of course.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.